In recent years, polymer microparticles are actively developed and widely used for various industrial applications. In particular, spherical polymer microparticles with narrow particle size distribution are applied to uses in filters, separation membranes, dispersants, powder coating, resin modifiers, coating agents, and the like, owing to their excellence in processability, fluidity, and surface properties. Although these polymer particles are made of various materials such as acrylic resin, styrene resin, melamine resin, and polyolefin, resin microparticles based on polyolefin, especially polyethylene, attract attention due to the advantages of high crystallinity, high melting temperature, and increased chemical stability. Using polyethylene-based microparticles, various new materials and new applications have been proposed and put into practical use, making use of their features such as water resistance, oil resistance, chemical resistance, and biological safety, which are not attained with other materials.
For example, polyethylene-based microparticles, non-treated or with treatment such as surface modification, can be used as filler of columns for separating chemical or biological substances with high efficiency, adsorbents or catalyst carriers with high specific surface area, and others. Moreover, they are utilized as carriers with function of delivery and sustained release of drugs, dispersing aid for homogeneously dispersing fine-particulate substances with low dispersibility, or highly safe particulate materials with good touch effect to skin in cosmetics.
In addition, polyethylene-based microparticles are actively investigated for applying to novel functional materials such as materials for separators in lithium batteries or lithium ion secondary batteries, materials for optical filters with functions of diffusion, reflection, or inhibition of reflection of light, high-performance binders of sintered porous materials such as ceramics, pore-forming materials in breathable films, carriers for immobilizing immunochemically active substances, microporous sintered filters with high specific surface area, skidding aids, toner, delusterants for paint, light diffusive additives, insulating fillers, nucleating agents for crystallization, fillers for chromatography, and carriers for immunodiagnostic agents.
Commonly to these uses as novel high-performance materials, for further functionalization and improvement in performances and quality, there is a demand for polyethylene-based spherical ultra-fine particles with smaller particle diameter and narrower particle size distribution free from inter-particle agglomeration.
On the other hand, currently known processes for manufacturing polyethylene-based microparticles are roughly classified into the following four techniques, i.e., (1) mechanical grinding (pulverization at room temperature or in frozen state, wet grinding, jet grinding), (2) spraying (dry, coagulation), (3) forced emulsification (melt emulsification, solution emulsification), and (4) suspension polymerization.
The mechanical grinding is a method of applying pulverization energy such as impulse force and shear force directly to bulk polymer to pulverize. The shape of microparticles obtained by this method generally tends to be indeterminate form, and therefore polyethylene-based microparticles with narrow particle size distribution are hardly obtained.
The spraying is a method of spraying a liquefied substance such as polymer solution made by dissolving bulk polyethylene in a solvent or melted polymer from a nozzle, followed by solidifying by drying/cooling to provide polymer microparticles. The polyethylene-based microparticles obtained by this method are formed as highly spherical microparticles due to surface tension of the liquefied substance sprayed, though they are often obtained in an agglomerated form of some particles and generally have widely distributed particle size. Moreover, depending on the molecular weight of polyethylene-based resin, the polymer solution may have high viscosity and cannot be sprayed in a particulate form because of stringiness or other adverse events on spraying. Therefore, it is hard to apply the spraying to high-molecular-weight polyethylene-based resin.
Meanwhile, the emulsification is a method of forcedly emulsifying polyethylene-based resin at temperatures not lower than its melting point in the presence of an emulsifier or dispersant in aqueous medium. This method has an advantage that, because of applied shear force to melted polymer in aqueous medium, the resultant polyethylene-based particles are more spherical and less agglomerated compared with the above two microparticle-forming methods. However, even with this method, as the molecular weight of polyethylene-based resin increases, it becomes more difficult to maintain narrow particle size distribution; thus it cannot be applied to polyethylene with ultra-high molecular weight. Moreover, this method is accompanied by problems such as remaining of the used emulsifier in the microparticles, and hence its use is restricted in some case.
The above microparticle-forming methods, because of using bulk polyethylene as a starting material, require two steps of processes: manufacturing polyethylene with an olefin-polymerization catalyst, by high-pressure radical polymerization, or otherwise; and forming microparticles therefrom. On the other hand, in forming microparticles by suspension polymerization, polyethylene-based microparticles can be directly obtained from ethylene monomer by polymerization using a solid olefin-polymerization catalyst formed into microparticles with controlled shape. In this method, so-called replica effect works, i.e., the particle shape and size distribution of solid olefin-polymerization catalyst are directly reflected in the particle shape and size distribution of resultant polyethylene-based microparticles; therefore, it is predicted that if the particles of solid olefin-polymerization catalyst used in polymerization are agglomerated, only polyethylene-based microparticles likewise agglomerated should be obtained.
Therefore, in order to obtain non-agglomerated, spherical polyethylene-based microparticles with narrow particle size distribution, a solid olefin-polymerization catalyst formed into microparticles with controlled shape is indispensable. A variety of such catalysts and synthetic methods thereof have been disclosed. For example, Japanese Patent Laid-Open Publication No. H05-320244 discloses a solid catalytic component obtained by contacting magnesium halide, tetraalkoxytitanium, aluminum halide, and an ether compound in a specific manner; and a method for manufacturing polyethylene using said solid catalyst. This document describes that the resultant polyethylene has very narrow particle size distribution, excellent flowability and bulk density. However, the polyethylene obtained has a diameter of 100 μm or greater and contains a slight amount of coarse particles sized 600 μm or greater, and hence it is substantially hard to regard as ultra-fine particles and unsatisfactory in the above uses as novel high-performance materials.
Japanese Patent Laid-Open Publication No. H05-301921 discloses a solid catalyst component synthesized by a specific contact method using diethoxymagnesium, tetrabutoxytitanium, tetrachlorosilane, aromatic dicarboxylic acid diester, and tetrachlorotitanium as raw materials. Japanese Patent Laid-Open Publication No. H07-41514 discloses a solid catalyst component synthesized by specific method using diethoxymagnesium, tetraisopropoxytitanium or tetrakis(2-ethylhexoxy)titanium, tetrachlorosilane, sorbitan fatty acid ester, tetrachlorotitanium, and an aromatic dicarboxylic acid diester as raw materials; and polyethylene manufactured by polymerization of ethylene using that solid catalyst component. It is described that these polyethylenes have narrow particle size distribution and high sphericity. However, the particle diameter of polyethylene is still 100 μm or more, which is unsatisfactory as ultra-fine particles desired to be sized tens of micrometers. Judging from the nominal particle diameter of solid catalyst, it is estimated that manufacturing ultra-fine polyethylene particles is substantially difficult.
In Japanese Patent Laid-Open Publication No. S60-163935, EP 0159110-B, and U.S. Pat. No. 4,972,035, it is disclosed that polyethylene microparticles with controlled particle diameter and particle size distribution can be manufactured by high-speed shearing of polyethylene particles obtained in polymerizing ethylene under specific conditions in the presence of a specific Ziegler-type catalyst, or by polymerization of ethylene with a specific fine-dispersed Ziegler catalyst that has been sheared at high speed. It is described that those polyethylenes are tens of micrometers in particle diameter, contain substantially no coarse particles, and have narrow particle size distribution. However, those materials contain a large amount of polyethylene particles agglomerated each other, and the particle shape is not spherical but rather confetto-like. For this reason, it is supposed that these materials should be poor in performances such as flowability, dispersibility, and packing characteristics as powder, and thus they are not sufficiently compatible with the above uses as novel high-performance materials. The interaction between agglomerated particles is partly based on strong chemical bonds, and hence it seems impossible to completely dissociate into non-agglomerated state by the high-speed shearing described in these publications or other mechanical cracking processes. Furthermore, in an attempt to obtain polyethylene microparticles with a smaller particle diameter, the cohesiveness between resultant polyethylene particles tends to increase, so that it is supposed that non-agglomerated ultra-fine particles, for example, with a particle diameter of around 10 μm are substantially impossible to manufacture.
According to the disclosure in Japanese Patent Laid-Open Publication No. 2004-143404, when a mixture of polyolefin-based resin and a fluid that does not dissolve that polyolefin-based resin at a normal temperature and pressure is heated and/or pressurized to make that fluid into super-critical or sub-critical state, followed by rapid cooling or pressure release of that fluid, spherical particles with a particle diameter of 1 μm or less are obtained.
However, this method required two steps of processes, manufacturing polyolefin particles and forming into microparticles, and because of the extremely small particle diameter, these particles have difficulty in handling on forming and impose problems relating to environmental pollution in the manufacturing processes or the safety in working environment.
For further functionalization and improvement in performances and quality, the microparticles of polyethylene-based resin are desired to have a smaller particle diameter and narrower particle size distribution and to be free from inter-particle agglomeration; it is, therefore, longed for the advent of a component of olefin-polymerization catalyst that enables manufacturing such spherical polyethylene-based microparticles, i.e., a magnesium-containing carrier serving as a carrier for the catalyst.
In some uses, polyethylene-based microparticles, after formed into a filter, a film or the like, are treated for surface functionalization by methods such as treatment with an oxidizing strong acid, plasma exposure, electron beam irradiation, laser irradiation, and UV exposure. Examples of these methods are disclosed in “Conspectus of Coating Technique in Plastics” (Industrial Technique Service Center, P. 251 (1989)).
Nevertheless, there has been no polyethylene-based microparticles material with uniform particle diameter and size having functional groups on the surfaces, and its discovery is awaited as a novel material.    [Patent Document 1] Japanese Patent Laid-Open Publication No. H5-320244    [Patent Document 2] Japanese Patent Laid-Open Publication No. H5-301921    [Patent Document 3] Japanese Patent Laid-Open Publication No. H7-41514    [Patent Document 4] Japanese Patent Laid-Open Publication No. S60-163935    [Patent Document 5] EP 0159110-B    [Patent Document 6] U.S. Pat. No. 4,972,035    [Patent Document 7] Japanese Patent Laid-Open Publication No. 2004-143404    [Non-patent Document 1] “Conspectus of Coating Techniques in Plastics” (Industrial Technique Service Center, p. 251 (1989))